In the Entrenchment beta, the computers used to lay mines around each of their gravity wells until they had thousands of mines per gravity well, and it slowed peoples' games down to a crawl.
In your case, even though there is a limit of 150 mines you can place per gravity well, they are still units that aren't present in the original game. So there's probably just more units that the CPU has to account for which is why your game might be slower.
